1. Field of the Invention
The present disclosure relates to a projection optical system, and an image projection device including a projection optical system.
2. Description of the Related Art
An LCD (liquid crystal display) projector is widely known as an image projection device. In recent years, with high-resolution images of liquid crystal panels and efficient energy control of light source lamps, the development of LCD projectors with increased brightness and lower prices is in progress. Small-size, light-weight image projection devices using a DMD (digital micro-mirror device) have spread, and such image projection devices are widely used at offices, schools, households, etc. Especially, front type projectors have good portability among the LCD projectors and may be conveniently used at small meetings or the like. Some examples of image projection devices will be described in the following.
For example, Japanese Laid-Open Patent Publication No. 2006-235516 discloses a projection optical system, and an image projection device using the projection optical system. The projection optical system is arranged to have a small projection distance. In the projection optical system, an intermediate image of the image forming element is formed by a first optical system including a lens group, and a light beam is greatly raised by a second optical system including a concave surface mirror, so that an image corresponding to the intermediate image is formed on a screen.
Japanese Laid-Open Patent Publication No. 2009-204846 discloses an image projection device in which a mirror is disposed between a concave surface mirror and a screen, and a light beam reflected from the mirror passes through a portion between a lens group and the concave surface mirror and is directed to a screen. The vertical positional relation of the screen to the optical axis of the lens group is equated to that of the concave surface mirror to the optical axis of the lens group.
Japanese Patent No. 4223936 discloses an image projection device in which a flat surface mirror is disposed between the optical path of a lens part and a mirror part of a projection optical system to bend the optical path about 90 degrees. The lens part is arranged upright to prevent the projection optical system from projecting to the screen surface.
Japanese Laid-Open Patent Publication No. 2009-116149 discloses an image projection device in which a projection optical system including a concave surface mirror is arranged. The image projection device includes a screen, a reflection optical part having a curved surface mirror disposed on the projection surface of the screen, a bending mirror disposed before the reflection optical part on the optical path and on the non-projection surface of the screen, a refraction optical part disposed before the reflection optical part on the optical path and on the non-projection surface of the screen, and an image forming optical part disposed on the non-projection surface of the screen.
However, in the image projection device disclosed in Japanese Laid-Open Patent Publication No. 2006-235516, if the image forming element, the first optical system, and the second optical system are arranged in the housing of the image projection device and an image is projected on the external screen, the housing projects over the screen surface, and there has been a problem that the image projection device has to be embedded in the wall. It has been difficult to obtain an optimal projection distance. Even if the image is projected, the resulting image may become out of focus.
In the projection optical system in the image projection device of Japanese Laid-Open Patent Publication No. 2006-235516, an intermediate image is formed with the lens group and imaging of the intermediate image is performed on the screen using the concave surface mirror, so that the image of the image forming element is generated. In this projection optical system, the image forming element is shifted relative to the lens optical axis and the light beam is raised by the concave surface mirror. As a result, the projection distance can be made short. Therefore, the position of the concave surface mirror to the lens optical axis is opposite to the position of the screen to the lens optical axis, and if the image is projected above the optical axis, the concave surface mirror must be arranged below the optical axis. Hence, there has been a problem that the housing must have a large thickness at its lower portion, and the overall housing must be bulky.
In the image projection device disclosed in Japanese Laid-Open Patent Publication No. 2009-204846, when the image projection device is arranged on a desk to project an image on a wall disposed above the desk, the projection plane (namely, a screen) must be disposed above the lens optical axis. Apart from the teaching of Japanese Laid-Open Patent Publication No. 2006-235516, the concave surface mirror is arranged above the optical axis. Hence, if the concave surface mirror can be folded up when the image projection device is not in use and can be restored in its original position when it is in use, then it is possible to make the housing compact at the time of non-use, and the problem that the overall housing is bulky can be avoided.
However, it is difficult to eliminate the problem that the image projection device must be embedded in the wall, when it is necessary to arrange the image projection device under certain conditions of the projection distance. If the projection distance can be made comparatively large, the light beam directed to the screen can pass through the intermediate portion between the lens and the concave surface mirror. However, if the projection distance is shortened (i.e., if the light beam is raised greatly), the size of the concave surface mirror must be large in the direction of departing from the optical axis and the power of the concave surface mirror is increased. The part that is located farthest from the optical axis of the concave surface mirror will interfere with the lens surface. Hence, there has been a problem that the light beam directed from the reflection mirror to the screen is interrupted by the concave surface mirror and cannot reach the screen.
In the image projection device disclosed in Japanese Patent No. 4223936, there is no need for arranging the image projection device by embedding the same in a wall. However, the image projection device has the structure in which the lens part stands upright. When the image projection device is arranged on a desk to project an image on a wall, the center of gravity of the image projection device arranged lies too high. There has been a problem that the image projection device easily falls on the desk. In such a case, the display surface of the image forming element faces upward and there has been a problem that the dust is easily deposited in the inside of the image projection device.
The image projection device disclosed in Japanese Laid-Open Patent Publication No. 2009-116149 is arranged such that the image projection device can be embedded in a wall. Japanese Laid-Open Patent Publication No. 2009-116149 does not suggest a method of arranging the image projection device without being embedded in the wall.